We have proposed the development of genetically engineered viral vectors that can selectively and efficiently infect and kill brain tumor cells in situ without harming surrounding brain cells and without causing systemic disease and have focused on conditionally-replicating mutants of herpes simplex virus-1 (HSV-1) which we have genetically engineered to be attenuated for neurovirulence. We developed the vector, G207, a multimutated HSV-1 that conditionally replicates in glioblastoma, malignant meningioma, and other tumors but is non-neuropathogenic in HSV-sensitive mice and subhuman primates. Safety and efficacy studies have been done to allow G207 to be considered for human trial. We have also demonstrated that G207 can induce a specific cell-mediate immune response to tumor cell surface antigens and that this can be boosted with the introduction of cytokines such as IL-12 into a defective vector grown with G207 as a helper virus. We now plan studies to optimize this treatment. We will study factors with possible adverse effects on the clinical use of HSV for brain tumor therapy such as the effects of co- treatment with steroids and the consequences of prior exposure to HSV on the efficacy of HSV-tumor therapy. Tumor models in mice will be used to test. The effects of exposure to steroids or of prior exposure and sero- positivity to HSV. In order to improve the specificity of targeted tumor cell destruction, we also construct and test transcriptionally targeted HSV vectors for selective destruction of cells expressing nestin or midkine. In order to improve the treatment of cells at a distance from virus inoculation, we will also explore the use of HSV vectors expressing cytokines, immune co-stimulatory molecules such as B7-1 and/or a suicide gene, or a combination of these in order to modulate the host immune system to optimize brain tumor therapy with HSV. Through our first grant, we have developed the first HSV vector that can be safely used for brain tumor therapy. Through these studies we expect to create the next generation of HSV vector with improved efficacy and selectivity.